[time-nuts] Fixing an HP 5370A with a missing ROM board

[Mark Sims]

I recently got in an HP5370A that had been robbed of the ROM card and the oven oscillator power supply card...  ah, crap...  Swapping the missing cards from a known good unit showed the counter was in otherwise excellent operating condition.

The HP5370A firmware is stored on eight 2708 1Kx8 EPROM chips on the ROM card.  The firmware is compiled to execute at addresses 7800-7FFF.  The system normally does not use address line A15 and the upper and lower halves of the address space are mirrored.  (BTW,  the system address bus is inverted polarity...  important to remember if you are looking at the code).

It turns out that the 5370A CPU card has an empty socket for a firmware ROM.  This socket supports a 4Kx8 ROM that will appear in the address space at C000-FFFF if switch 3 on the CPU card is turned on.  Hmmm,  with a little modification it should be possible to install an 8Kx8 EPROM there with a copy of the firmware.  

Motorola did make the MCM68764 and MCM68766 8kx8 EPROMS in a 24 pin package.  They are hard to find (but are on Ebay for 4 bucks each) and most programmers do not support them.  The EPROM A12 address line is on a pin tied to +5V on the CPU board socket (pin 21).  You could easily patch the board and use the Motorola chip.

I have a pile of 27C512 (64Kx8) EPROMS in a 28 pin DIP package.   Most of the pins match the CPU board ROM socket connections so I chose to use them.  To use the 27C512 chip,  first I read in each of the eight 2708 chips from a good 5370A ROM board using a Data I/O Unisite programmer (note that the DIO 2900 and 3900 series programmers don't support 2708's!!!).  Next I used the DOS COPY command to concatenate the 8 1Kx8 ROM images into a single 8Kx8 ROM image file (remember to use the /B binary file parameter!):
   COPY ROM1/B+ROM2/B+ROM3/B+ROM4/B+ROM5/B+ROM6/B+ROM7/B+ROM8/B  IMAGE.ROM/B

Finally,  I concatenated 8 copies of the 8Kx8 image into a 64Kx8 ROM image which I then programmed into the 27C512.  You could just put one copy of the 8K ROM image into the upper 8K of the EPROM,  but this was easier to do.

Lift pins 23 (A11) and 20 (/OE) on the 27C512 so that they won't go into the EPROM socket.  Solder a jumper wire between EPROM pins 20 (/OE) and 22 (/CS).  Be careful not to short to pin 21.  This connects the /CS and /OE pins of the EPROM.  Solder a jumper wire between EPROM pins  1,28,27,26.  This puts +5V on these pins (A13,A14,A15,VCC).  Install the EPROM in the CPU board socket.  Pins 1,2,27, and 28 of the EPROM chip should be hanging in the air.  

Solder a jumper wire between pin 23 of the EPROM chip (A11) and U14-7 on the CPU board (on the photo,  I ran this wire to the back of the board and picked up the signal on pin 18 of the 24 pin the EPROM socket)  Solder another jumper wire between pin 2 of the EPROM chip (A12) and U16-9 on the CPU board.  This connects the CPU address bus A11 and A12 signals to the EPROM.  (you should first verify that the CPU board pin numbers mentioned match the revision/layout of your CPU board)

Next,  you have to modify the CPU board to properly address the new EPROM chip.  Cut pin 3 of U6 on the CPU board flush with the board,  lift the nub of the pin up so it no longer touches the board (this pin normally connects to A15).  Solder a jumper to the lifted pin and pin U16-11 (A13) on the CPU board.  This modifies the CPU board so that the CPU external data bus buffer will be disabled and the EPROM chip will be enabled when the external ROM board address space is addressed (i.e. A13 and A14 high instead of A15 and A14 high).  

Turn on DIP switch 3 on the CPU board to enable the EPROM socket.  If all goes well,  you should have a 5370A working without the ROM board (just like the 5370B does).

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